The present invention relates to sector molds for tires, particularly molds such as described in U.S. Pat. No. 4,289,463.
This type of mold comprises a lower molding part, an upper molding part, and a series of sectors to assure the molding of one side wall, of the other side wall and of the tread of the tire, respectively. Furthermore, the mold comprises a peripheral hoop which surrounds the sectors, causing their radial closing movement (radial movement of all the sectors towards the center) and holding them firmly closed in order to withstand the molding pressure. The opening of the mold, in addition to a suitable kinematic system, employs the action of springs in order to return the sectors, that is to say, to move them away from the center of the mold, and in order to cause them to swing slightly. For further details, reference may be had to the above mentioned patent.
The advantage of this type of mold over container molds in which the sectors accompany the upper molding part which lifts up upon the opening of the mold resides in the fact that the weight to be lifted in order to open the mold is much less since the entire tread molding part remains on the lower plate of the press, in the same way as the lower molding part. Furthermore, the improvement proposed in the said patent takes up less diametrical space. Now, for the same space, it makes it possible to mold tires of larger diameter. In fact, the swinging of the sectors makes it possible to reach more rapidly a diameter of release which is sufficient to remove the vulcanized tire.
Many tread patterns molded on the treads of tires, in particular tires for passenger cars, comprise numerous sipes. This makes it necessary to assure a purely radial movement of recoil upon the removal from the mold in order to limit, as far as possible, the mold-removal forces imposed on the tire and on the inserts molding the sipes. Now, in the mold which is the subject of said patent, the radial recoil springs by themselves assure the opening movement of the series of sectors. The opening takes place in the following manner: The release of the peripheral hoop is not accompanied by any movement of the molding parts. The moving away of the upper molding part permits the swinging of the sector under the action of the spring or springs interposed between the lower molding part and each sector; the sectors being all released from the tread, they then all move apart under the action of the radial recoil springs.
With this known mold, it is impossible to cause the radial recoil before the swinging. In fact, the force for tearing a sector away from the vulcanized tread is very great, since there is a sticking effect of the sector on the tread. It is not possible to install radial recoil springs which are sufficiently strong to overcome this sticking in all cases which may be encountered. The theoretical spring necessary to develop sufficient force would be too large.
In sector molds of purely radial recoil, the recoil movement is caused by sliderails of the "dovetail" type and this type of control is incompatible with the swinging of the sector. The invention is directed at reconciling the purely radial movement of recoil upon the phase of removal from the mold of the tread pattern taking place at the very beginning of the recoil, with a subsequent swinging of the sectors in order to take up as little space as possible.